The invention relates generally to magnetic resonance imaging (MRI) systems and more specifically to a gradient amplifier system adapted for use in MRI systems.
In just a few decades, the use of magnetic resonance imaging (MRI) scanners has grown tremendously. MRI scans are being increasingly used to aid in the diagnosis of multiple sclerosis, brain tumors, torn ligaments, tendonitis, cancer, strokes, and the like. As will be appreciated, MRI is a noninvasive medical test that aids physicians in the diagnoses and treatment of various medical conditions. The enhanced contrast that an MRI scan provides between the different soft tissues of the body allows physicians to better evaluate the various parts of the body and determine the presence of certain diseases that may not be assessed adequately with other imaging methods such as X-ray, ultrasound, or computed tomography (CT).
A conventional MRI system typically establishes a homogenous magnetic field generally along a central axis of a subject undergoing an MRI procedure. This homogeneous magnetic field affects the gyro magnetic material of the subject for imaging by aligning the nuclear spins, in atoms and molecules forming the body tissue. For example, in medical applications, the nuclear spins are aligned along the direction of the magnetic field. If the orientation of the nuclear spins is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their spins with the field. Perturbation of the orientation of nuclear spins is typically caused by application of radio frequency (RF) pulses tuned to the Larmor frequency of the material of interest. During the realignment process, the nuclei precess about their axes and emit electromagnetic signals that may be detected by one or more RF detector coils placed on or about the subject.
The frequency of the magnetic resonance (MR) signal emitted by a given precessing nucleus depends on the strength of the magnetic field at the location of the nucleus. It is possible to distinguish signals originating from different locations within the subject by using encoding, typically phase and frequency encoding, created by gradient coils that apply gradient fields over the main magnetic field. A typical MRI system includes three gradient coils for providing respective fields along the X, Y and Z axes. Control of the gradient coils allows for orientation of the axes for encoding of locations within the subject, and for selection of a desired “slice” for imaging.
Furthermore, these gradient coils typically produce additional magnetic fields that are superimposed on the primary magnetic field to permit localization of the image slices and also provide phase encoding and frequency encoding. This encoding permits identification of the origin of resonance signals during image reconstruction. The image quality and resolution depends significantly on how the applied fields can be controlled. To achieve faster imaging rates, the gradient fields are typically modified at frequencies of several kHz. Control of the gradient coils is generally performed in accordance with pre-established protocols or sequences, called pulse sequence descriptions, permitting many types of tissues to be imaged and distinguished from other tissues in a medical context, or for imaging various features of interest in other applications.
Typically, a gradient coil operates at about 500 amperes of current and at a voltage in a range from about 1000 volts to about 2000 volts. Therefore, it is desirable to provide a gradient amplifier that is configured to supply the gradient coils with the desired current and voltage levels. In certain embodiments, the gradient amplifier is a power amplifier.
Earlier implementations of gradient amplifiers used linear amplifiers that provided high fidelity. However, given present power level requirements, the use of these amplifiers becomes impractical due to need for the higher voltages and currents. Present day techniques use hybrid systems that combine linear amplifiers with switching power stages. Such systems use bridges in parallel or bridges stacked to meet the system requirements, and typically employ power semiconductor devices. However, in the conventional gradient amplifier system, each of the bridges has different direct current (DC) link voltages and different voltage commands, which unfortunately results in different switching frequencies for each of the bridges. Since each of the bridges operates at different switching frequencies with different DC link voltages, there is significant power loss in the gradient amplifier system. Moreover, the power loss increases when a low voltage is desired across the gradient coil. In addition, the power losses are unevenly distributed across the bridges, and the loading on each bridge is also different, causing intense thermal stress on the gradient amplifiers.
It is therefore desirable to develop a design of a gradient amplifier system that reduces power loss. Particularly, it is desirable to develop the design of a controller stage and a power stage architecture in the gradient amplifier system that provides high power and delivers high fidelity with reduced power loss and cost through circuit topologies and control mechanisms.